IC-736 mic input question
 

I'm struggling with poor TX audio quality on my Icom 736, bought
used several months ago. The mikes I use (an included HM-35, a
homebrewed preamped mike, a Radio Shack amplified mike) all get
reports of audio with no lows, pinched, not terrible but worse
than from other similar radios (e.g. an IC 735 with the same HM).
It might be good for pileups, but it's uncomfortable for normal
ragchewing.

I've read a lot about insufficient mike gain in older Icoms, and
about the possibility of plugging into the acc socket in the back.
But I'd like to first determine whether my mike circuitry might
have a problem.

I put a scope on the mic in and mic gnd pins of the connector,
and on top of the normal 7 or so V DC (for electret biasing) I
see an almost 1 V p-p sine wave, about 110 kHz, and very "noisy".
The trace gets clean and sharp after shunting the mic ground to
the regular chassis ground with a 10 nF cap. It's there whether
the mike is plugged in or not, and whether the radio is receiving
or transmitting.

This high frequency signal should not interfere with the audio,
but who knows. It might also indicate some other problem, e.g.
the built-in switching supply bleeding noise into the rest of
the radio.

I haven't noticed any other glitches with this rig whatsoever. I
do not have any excessive birdies from the PS. So I'm looking for
ideas/opinions. Ideally, some kind OM with a 736 might be able to
see whether his has that strange signal on the mic input too, in
which case I could rest assured that it isn't unique to my radio.

Thanks a lot in advance for any hints or advice. 73, KC9DUX

--
Eric Behr | NIU Mathematical Sciences | (815) 753 6727
| http://www.math.niu.edu/~behr/ | fax: 753 1112

Have you tried a different mike on the same rig?
Might be a cable/shield problem, loose connection or a fubar'd
element. ( it's usually a fubar'd cable)
I built my own mike out of junkbox parts for a 735, seems to work
OK..uglier than sin though!
G'Luck!

On 25 Feb 2005 17:33:59 GMT, (Eric Behr) wrote:

I'm struggling with poor TX audio quality on my Icom 736, bought
used several months ago. The mikes I use (an included HM-35, a
homebrewed preamped mike, a Radio Shack amplified mike) all get
reports of audio with no lows, pinched, not terrible but worse
than from other similar radios (e.g. an IC 735 with the same HM).
It might be good for pileups, but it's uncomfortable for normal
ragchewing.

I've read a lot about insufficient mike gain in older Icoms, and
about the possibility of plugging into the acc socket in the back.
But I'd like to first determine whether my mike circuitry might
have a problem.

I put a scope on the mic in and mic gnd pins of the connector,
and on top of the normal 7 or so V DC (for electret biasing) I
see an almost 1 V p-p sine wave, about 110 kHz, and very "noisy".
The trace gets clean and sharp after shunting the mic ground to
the regular chassis ground with a 10 nF cap. It's there whether
the mike is plugged in or not, and whether the radio is receiving
or transmitting.

This high frequency signal should not interfere with the audio,
but who knows. It might also indicate some other problem, e.g.
the built-in switching supply bleeding noise into the rest of
the radio.

I haven't noticed any other glitches with this rig whatsoever. I
do not have any excessive birdies from the PS. So I'm looking for
ideas/opinions. Ideally, some kind OM with a 736 might be able to
see whether his has that strange signal on the mic input too, in
which case I could rest assured that it isn't unique to my radio.

Thanks a lot in advance for any hints or advice. 73, KC9DUX

In article ,
bart wrote:
Have you tried a different mike on the same rig?
Might be a cable/shield problem, loose connection or a fubar'd
element. ( it's usually a fubar'd cable)

Yes, that signal shows up with 3 different mikes, or with no mike
plugged in at all... It's definitely from the radio. Thanks - E.

--
Eric Behr | NIU Mathematical Sciences | (815) 753 6727
| http://www.math.niu.edu/~behr/ | fax: 753 1112

Hmm, strange.
Try tracing it through the schematic?
manuals here;
http://www.mods.dk/view.php?ListManuals=icom
http://www.marcucci.it/english/download/
( you have to use IE to D/L)
there's an article on the DDS here..I'm not sure if it's related at
all;
http://www.mods.dk/view.php?ArticleId=257


On 26 Feb 2005 15:01:35 GMT, (Eric Behr) wrote:

In article ,
bart wrote:
Have you tried a different mike on the same rig?
Might be a cable/shield problem, loose connection or a fubar'd
element. ( it's usually a fubar'd cable)

Yes, that signal shows up with 3 different mikes, or with no mike
plugged in at all... It's definitely from the radio. Thanks - E.

In article ,
bart wrote:
Hmm, strange.
Try tracing it through the schematic?
manuals here;
http://www.mods.dk/view.php?ListManuals=icom
http://www.marcucci.it/english/download/
( you have to use IE to D/L)
there's an article on the DDS here..I'm not sure if it's related at
all;
http://www.mods.dk/view.php?ArticleId=257

Thanks again, I'll be poring over the diagram.

--
Eric Behr | NIU Mathematical Sciences | (815) 753 6727
| http://www.math.niu.edu/~behr/ | fax: 753 1112

On 25 Feb 2005 17:33:59 GMT, (Eric Behr) wrote:

I'm struggling with poor TX audio quality on my Icom 736, bought
used several months ago. The mikes I use (an included HM-35, a
homebrewed preamped mike, a Radio Shack amplified mike) all get
reports of audio with no lows, pinched, not terrible but worse
than from other similar radios (e.g. an IC 735 with the same HM).
It might be good for pileups, but it's uncomfortable for normal
ragchewing.

I've read a lot about insufficient mike gain in older Icoms, and
about the possibility of plugging into the acc socket in the back.
But I'd like to first determine whether my mike circuitry might
have a problem.

I put a scope on the mic in and mic gnd pins of the connector,
and on top of the normal 7 or so V DC (for electret biasing) I
see an almost 1 V p-p sine wave, about 110 kHz, and very "noisy".
The trace gets clean and sharp after shunting the mic ground to
the regular chassis ground with a 10 nF cap. It's there whether
the mike is plugged in or not, and whether the radio is receiving
or transmitting.

This high frequency signal should not interfere with the audio,
but who knows. It might also indicate some other problem, e.g.
the built-in switching supply bleeding noise into the rest of
the radio.

I haven't noticed any other glitches with this rig whatsoever. I
do not have any excessive birdies from the PS. So I'm looking for
ideas/opinions. Ideally, some kind OM with a 736 might be able to
see whether his has that strange signal on the mic input too, in
which case I could rest assured that it isn't unique to my radio.

Thanks a lot in advance for any hints or advice. 73, KC9DUX

If you haven't already, you might check Heil's page on Icom mikes:

http://www.heilsound.com/amateur/icom_fix.htm

(or just e-mail 'em direct)

bob
k5qwg

In article ,
Bob Miller wrote:
On 25 Feb 2005 17:33:59 GMT, (Eric Behr) wrote:

I'm struggling with poor TX audio quality on my Icom 736, bought
used several months ago.

If you haven't already, you might check Heil's page on Icom mikes:
http://www.heilsound.com/amateur/icom_fix.htm
(or just e-mail 'em direct)

Godd idea, I'll email them. I'd seen their Icom page, but thanks
a bunch anyway.

--
Eric Behr | NIU Mathematical Sciences | (815) 753 6727
| http://www.math.niu.edu/~behr/ | fax: 753 1112

Hi Eric

The BFO adjustment for USB & LSB is out of alignment. This determined the carrier point and the SSB
Audio frequency response. Check your owners manual it will tell you how to align it. I have repaired
many. That's your problem with the radio !!

73 John